Material discriminating device for metallic plates

ABSTRACT

A material discriminating device for metallic plates comprises a base member having a passage with dimensions fitting the configuration of metallic plates whose material is to be discriminated, pressing means disposed along the passage for applying a predetermined load to the metallic plates, and deformation detecting means disposed along the passage for detecting the magnitude of the permanent deformation of the metallic plates caused by the pressing means, whereby the material of metallic plates may be discriminated on the basis of the magnitude of the permanent deformation which depends upon the elastic limits of the metallic plates. Novel constructions of the pressing means as well as the deformation detecting means are also provided.

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States atet [191 Entlo et al.

MATERHAL DiSCRlMliNATlNG DEVICE FOR METALLIC PLATES Inventor's: Talkashi Endo, Kobe; Riichiro Yamashita, Kakogawa, both of Japan Mitsubishi Jukogyo Kabushiki Kaisha, Tokyo, Japan Filed: Feb. 24, 1972 Appl. No.: 228,994

Assignee:

Foreign Application Priority Data Mar. 27, 1971 Japan 46-17747 Mar. 27, 1971 Japan 46-17751 Mar. 27, 1971 Japan 46-21439 US. Cl. 73/100 int. Cl. GOln 3/20 Field of Search 73/100, 89, 88 R; 209/79 PRESS/N6 DEV/CE;

2,404,584 7/1946 Liska et a1. 73/100 X Primary Examiner-Jerry W. Myracle Attorney, Agent, or Firm-McClew and Tuttle [5 7] ABSTRACT A material discriminating device for metallic plates comprises a base member having a passage with dimensions fitting the configuration of metallic plates whose material is to be discriminated, pressing means disposed along the passage for applying a predetermined load to the metallic plates, and deformation detecting means disposed along the passage for detecting the magnitude of the permanent deformation of the metallic plates caused by the pressing means, whereby the material of metallic plates may be discriminated on the basis of the magnitude of the permanent deformation which depends upon the elastic limits of the metallic plates. Novel constructions of the pressing means as well as the deformation detecting means. are also provided.

7 Claims, 7 Drawing Figures PATENTEDAPR is m4 SHEET 3 0F 3 FIG. 6

AND

DISCRIMIN- A) Tag C/RCU/ 7 FIG. 7

BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates in general to a material discriminating device for metallic plates, and more particularly to a such a device which discriminates material of metallic plates on the basis of the magnitude of permanent deformation of metallic plates caused by a predetermined load applied thereto.

Heretofore, as an automatic non-destructive testing device for checking whether or not a metallic plate stamped into a desired configuration such as, for example, a circular plate, is made of desired material, there were devices in which the electric resistance of the metallic plate was detected in comparison with a standard value of electric resistance according to an electromagnetic induction method, and on the basis of the de tected resistance value the material of the metallic plate was'discriminated.

However, such a material discriminating device in the prior art hifd the disadvantage that even if the electric resistance values detected for the respective metallic plates according to the electromagnetic induction method as described above where the same, the metallic plates would possibly have different alloy compositions consisting of metal components having different specific resistances, respectively. More particularly, in spite of the fact that the specific metallic plate could be made of many kinds of materials having the same resistance value, even with different alloy compositions for the respective materials, the above-referred prior art devices would deem the materials having the same resistance value as being the same material, and therefore, the prior art devices had the disadvantage that, from the observed results, it could not be definitely determined whether or not the observed metallic plate is made of a specific material, and thus it was impossible to exactly discriminate the material of the metallic plates.

Therefore, one object of the present invention is to provide a novel device which can discriminate the material of metallic plates exactly and with high precision.

Another object of the present invention is to provide a novel device which is adapted to give a deformation to a metallic plate and to discriminate the material of the metallic plate with high precision on the basis of whether or not the deformation comes within an elastic limit of the material forming the metallic plate.

Yet another object of the present'invention is to provide a novel device which can reliably discriminate the material of metallic plate without being affectedby the alloy composition and/or the co-existence of impurity in the metallic plate.

A further object of the present invention is to provide a novel device which can automatically and nondestructively discriminate the material of metallic plates.

A still further object of the present invention is to provide a material discriminating device, which is adapted to apply a load to a metallic plate and to auto matically discriminate the material of the metallic plate by detecting the pennanent deformation caused by the load, and in which the means for achieving the operation steps are specifically constructed and assembled together.

BRIEF DESCRIPTION OF THE DRAWINGS Now the present invention will be described in connection with its preferred embodiment illustrated in the accompanying drawings, in which:

FIG. 1 is a diagram showing the relation between the load applied to a metallic plate and the magnitude of deformation for explaining the principle of the material discrimination utilized by the device according to the present invention,

FIG. 2 is a schematic structural view illustrating one preferred embodiment of the device according to the present invention,

FIG. 3 is a longitudinal cross-section view of the automatic pressing device for the metallic plate as illustrated in FIG. 2,

FIG. 4 is a diagram showing the variation with time of the attractive force exerted by a solenoid in the automatic pressing device,

FIG. 5 is a plan view of the deformation measuring device for the metallic plate as illustrated in FIG. 2,

FIG. 6 is a side elevation view of the device illustrated in FIG. 5, and

FIG. 7 shows an electric circuit associated with the device illustrated in FIGS. 5 and 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT Now explaining the principle of the device according to the present invention with reference to FIG. 1, curves A and B represent the relation between the load and the magnitude of deformation under the load with respect to a hard metallic material A and a soft metallic material B, respectively, as classified according to the nature of materials.

Points A and B represent the respective elastic limits and, under a load lower than these points A and B the deformation can be restored to zero when the load is removed. Points A and B represent the respective breaking points.

If a load M that is higher than the elastic limit point B for the metallic material B but lower than the elastic limit point A for the metallic material A is applied to the respective materials A and B, then deformations corresponding to the points P and Q are caused in the respective materials. When this load M is removed, the deformation of the metallic material A is restored to zero because the load M is within the elastic limit, but the deformation of the metallic material B is reduced to and settled at the point N, passing through the hysteresis curve O N, because the deformation caused by the load M exceeds the elastic limit point B In other words, a permanent deformation of Ax would remain in the metallic material B. Therefore, if the relation between the load and the magnitude of deformation for the metallic material which is to be discriminated is preliminarily determined (for example, as shown by the curve OA A and if the deformation of a metallic material after a predetermined load M within the elastic limit has been applied thereto and then removed therefrom is observed, it is possible to discriminate whether or not the observed metallic material consists of the desired metallic material.

On the basis of this principle, the discriminating device illustrated in FIG. 2 is constructed. In FIG. 2, reference numeral (1) designates a base member provided with a channel-shaped passage (11') having a predetermined depth, the cross-section of which is shown on the right side of FIG. 2, and which is adapted to the configuration of the metallic plate whose material is to be discriminated, while reference numeral (2) designates a metallic plate whose material is to be discriminated and having a thickness equal to the depth of the passage (1) in the base member (1), plate (2) being illustrated as a circular metallic plate made of hard material in the drawings.

The metallic pl ate (2) is transferred along the passage (1) in the dii'ection of the arrow A shown in FIG. 2. Reference numeral (3) designates a pressing device for applying a predetermined load M to this metallic plate (2) while being transferred, and this pressing device gives a deformation of such magnitude as necessitated for the discrimination, to the metallic plate (2). Reference numeral (4) designates a deformation detecting device, which detects whether or not the metallic plate (2) once applied with the load M, retains a permanent deformation caused by the load M.

By the aid of the device as described above, it is possible to discriminate whether or not the metallic plate (2) is made of predetermined material through the step of applying a load M to the metallic plate (2) while being transferred along the passage (1) in the base member (1) by means of the pressing device (3) and then detecting by means of the deformation detecting device (4) whether or not a permanent deformation has been caused in the metallic plate (2) by load M.

Now the construction of the pressing device (3) will be described in more detail with reference to FIG. 3. In this figure, reference (D) designates a conveyer belt disposed along the passage (1) so as to frictionally engage with the upper surface of the metallic plate (2), and driven in the direction of the arrow (E), for transferring the metallic plate (2) along the passage (1) in the base member (1).

The base member (1) is provided with a slot (1") at the position where the pressing device (3) is disposed, and above and below slot (1") are provided a depressing rller (5) and a support roller (C), respectively, in an opposed relationship. The support roller (C) is fixedly disposed at the illustrated position and its shaft (6) is supported via a ball-and-roller bearing (7). On the other hand, the depressing roller (5) has its shaft (8) coupled to an appropriate driving device (not shown) and thereby it is rotationally driven in the direction of the arrow (F) in FIG. 3. Still further, this depressing roller (5) has a structure such that it may be moved vertically without being rocked horizontally, as described in the following. In more detail, the shaft (8) is carried by a support member (9), and a support rod (10) projecting vertically from the upper surface of support member (9) extends through a guide cylinder (11). The top portion of the support rod (10) is connected to a driving shaft (13) of a solenoid (12) by means of a bolt-nut assembly (14). On the other hand, the solenoid (12) comprises an iron armature (15) to which the driving shaft (13) is fixedly secured, a stator iron core (17) and a coil (18) wound around the stator iron core (17).

When no current is flowing through the coil (18), the armature (15) is lifted up by the resilient restoring force of a spring (19). Also, at the position where the support roller (C) and the depressing roller (5) are opposed to each other, there is disposed a non-contact type of position detector (20) for detecting the fact that the metallic plate has been transferred up to the substantially center position of the rollers (C) and (5). The position detector (20) feeds a detection output signal to an amplifier (21), whose output is connected to the coil (18) of the solenoid (12) via a current control circuit (22), which responds to the output signal of the amplifier (21) to make a switching operation for momentarily feeding a current to the coil (18) of the solenoid (12).

The magnitude of the current fed to the solenoid (12) is adjusted so as to apply a predetermined load to the metallic plate (2). In addition, the gap distance between the opposed surfaces of the support roller (C) and the depressing roller (5) is preset so that said gap distance may be less by AT than the thickness of the metallic plate (2) when the metallic plate (2) is not present in this opposed gap space.

In the pressing device (3) having the structure as described above, when the metallic plate (2) transferred along the passage (1') in the base member (1) by means of the conveyer belt (D) enters the gap space between the support roller (C) and the depressing roller (5), the depressing roller (5) is forcibly raised by AT. Therefore, the armature (15) is forcibly raised by AT via the driving shaft (13). Thereby the metallic plate (2) is lightly pressed against the support roller is (C) by the depressing roller (5) with a pressing force P K T which is determined by the elastic constant K of the resilient member (19) and the displacement AT.

When the metallic plate (2) has reached the substantially central position in the gap space between the support roller (C) and the depressing roller (5), the noncontact type of position detector (20) generates a signal.

This signal is amplified by the amplifier (21) and then applied to the current control circuit (22) which achieves a switching operation to provide a predetermined electric current flow through the coil (18). Therefore, the stator iron core (17) is magnetized, so that the armature (15 is attracted towards the stator iron core (17). Due to this attracting effect, the depressing roller (5) is depressed towards the support roller (C) by the intermediary of the driving shaft (13). In other words, the depressing roller (5) compresses the metallic plate (2), and thereby the metallic plate (2) is subjected to flexu 1 deformation.

The magnitude of the force transmitted to the depressing roller (5) due to the attraction of the armature iron core (15) towards the stator iron core (17), is varied in accordance with the travelling of the metallic plate (2) as shown in FIG. 4, where the attractive force is represented by P(t). In this figure, P, represents the initial compression force caused by the resilient member (19), this initial compression force being far smaller than the attractive force P(t). Since the effective time of the force P(t) is only momentary, the travelling of the metallic plate (2) is not prevented at the position where the roller (C) and the roller (5) are opposed to each other.

By the aid of the above-described pressing device (3), it is possible to apply a desired amount (that is determined by controlling the magnitude of electric current passing through the solenoid (12)) of flexual load momentarily to the travelling metallic plate (2).

Now the construction and function of the deformation detecting device (4) shown in FIG. 2 will be described with' reference to FIGS. 5, 6 and 7.

The previously described conveyer belt (D) is disposed at a position displaced leftwardly from the center line of the passage (1) as viewed in FIG. 5 and, at the detecting position (G-G) a depressing force is applied onto the metallic plate (2) via the conveyer belt by the action of the depressing pulley (24) disposed on the same line as the conveyer belt (D). Reference numeral (25) represents a tension spring for generating the pressing force of the depressing pulley or roller (241) against the metallic plate (2), numeral (26) represents a fulcrum bearing, and numerals (27) and (28) represent a bolt and an adjusting nut, respectively, for adjusting the height of the depressing pulley (24).

Detecting coils (29) and (30) are disposed so as to align with the line (GG) at the detecting position along the passage (1'), and also symmetrically with respect to the center line of passage (1'). The magnetic fluxes emanating from the detecting coils (29) and (30) are arranged so as to be substantially perpendicular to the metallic plate (2). In addition, for the purpose of generating a pulsed voltage at the time instant when the metallic plate (2) has reached the detecting position (G-G'), a position detector (23), consisting of a semiconductor photoelectric transducer element or the like, is provided at a position within the circumference of the metallic plate (2) in the proximity of its front edge as viewed in FIG. 5.

Here it is assumed that the metallic plate (2), which was preliminarily pressed by means of the abovedescribed pressing device (3), has been transferred-to the position of the detecting device (4) having the aforementioned construction by the action of the conveyor belt (D) from up to down as viewed in FIG. 5, while retaining its permanent deformation without changing its orientation with respect to the direction of the passage (1), that is, while maintaining the state where the metallic plate (2) is bent symmetrically with respect to the center line of the passage (1'). This state of the bent metallic plate (2) is shown by a broken line in FIG. 6, whereas a metallic plate (2) that is free of permanent deformation is shown by a solid line.

Thus the metallic plate (2) having the permanent deformation is brought to the state where the side of the metallic plate opposite to the conveyer belt (D) with respect to the center line of the passage (1') is raised due to the depressing force exerted by the conveyer belt (D) and pulley 24. This amount of rising is represented by AX. if the impedances of the detecting coils (29) and (30) are equal to each other and amount to Z,,, the increment of this impedance varying in accordance with the material and the physical nature of the metallic plate (2) being represented by AZ and the increment of the impedance varying in accordance with the distance between the detecting coil (29) or (30) and the metallic plate (2) is represented by A2,,

,then the impedances Z, andZ of the detecting coils (29) and (30), respectively, when the metallic plate (2) and the fluxes of the detecting coils (29) and (30) cross with each other, are indicated by the'following equations:

Z, Z AZ Z, Z, A2,, AZ

In other words, though the impedances Z, and Z, of the detecting coils (29) and (30) are both affected by the material of the metallic plate (2), the impedance Z, of

the detecting coil (29) does not involve the increment of impedance AZ representing the effect of the distance change between the metallic plate (2) and coil (29) that might becaused by the flexual deformation, owing to the one-side pressing effect of the depressing roller (24). Consequently, if a circuit arrangement is constructed so that the difference between the impedances Z, and Z of the detecting coils (29) and (30) may be detected, then the impedance increment AZ caused by the deformation of the metallic plate (2) can be detected and thus the magnitude of deformation AX can be also detected. The circuit arrangement for such a purpose is shown in FIG. 7.

In FIG. 7, reference numeral (33) designates an A.C. power supply, across which are connected the detecting coils (29) and (30) in series, and also across which are connected resistors (31) and (32) in series. The junction point H between the detecting coils (29) and (30) and the junction point J between the resistors (31) and (32) are respectively connected to input terminals of a discriminator circuit (34) as described later. The output of discriminator circuit (34) and the output of the aforementioned position detector (23) consisting of a semiconductor photoelectric transducer element or the like are connected to the respective inputs of an AND circuit (35).

1f the resistors (31) and (32) are adjusted so that the output voltage between the junction point H for the detecting coils (29) and (30) and the junction point J for the resistors (31) and (32) in FlG. 7 may become zero under the state where a metallic plate (2) made of a standard material and subjected to no deformation is opposed to the detecting coils (29) and (30), then an output voltage AE that is proportional to the magnitude of deformation AZ of the metallic plate (2) will be produced between junction points H and J when a bent metallic plate (2), as shown by a broken line in FIG. 6, is passing over the detecting coils (29) and (30).

Only when the output voltage AE exceeds a predetermined reference value, a fixed voltage is produced at the output of the discriminating circuit (343), and simultaneously therewith when the metallic plate (2) has reached and is passing over the position (G-G'), the front edge of the metallic plate (2) intercepts the position detector (23), resultingin a pulsed voltage (a sampling pulse). Since this pulsed voltage is generated every time the metallic plate (2) is passing over, when the fixed voltage appears at the output of the discriminator circuit (34), that is, when the deformation of the metallic plate (2) exceeds the predetermined amount, the AND circuit (35) is actuated to produce a signal at the output of said AND circuit, and thereby it can be discriminated that the amount of deformation of the metallic plate (2) exceeds a certain reference value. In other words, it is possible to determine that the metallic plate (2) under investigation is a metallic plate made of a material different from the standard metallic material.

Since the above-described deformation detecting device (4) operates according to an electromagnetic method, it is possible to discriminate the material of a metallic plate (2) travelling at a high speed.

Furthermore, if the above-described material discriminating device for a metallic plate is used in combination with an apparatus for comparing electric resistances on the basis of the electromagnetic induction method, discrimination of materials with higher precision can be achieved.

It is a matter of course that the above-described preferred embodiment of the invention has been given only by way of example and not in a limiting sense, and that various changes or modifications could be made to the construction and components of the illustrated device within the scope of the invention as specifically defined by the following claims.

What is claimed is:

l. A material discriminating device for metallic plates comprising an elongated base member having a passage of such dimensions as conform to configuration of plates whose material is to be discriminated as the plates are moved along said passage, pressing means positioned at a first point along said passage and operable, only when activated, to apply a predetermined load to metallic plates passing said first point, deformation detecting means positioned at a second point along said passage and operable, only when activated, to detect the magnitude of any permanent deformation of the metallic plates, passing said second point, caused by said pressing means, first plate detector means operable, responsive to detection of a plate at said first point, to activate said pressing means only when a plate is passing said first point, and second detector means operable responsive to detection of a plate at said second point, to activate said deformation detecting means only when a plate is passing said second point, whereby the material of metallic plates moving along said passage may be discriminated on the basis of the magnitude of the permanent deformation which depends upon the elastic limit of the metallic plates.

2. A material discriminating device for metallic plates, as claimed in claim 1, in which the base surface of said passage is substantially flat and planar, said deformation detecting means comprising a conveyor belt extending along said passage at a location spaced substantially laterally to one side of the longitudinal center line of said passage, said conveyor belt being engageable with metallic plates to move the same along said passage; a second pressing roller mounted for rotation in a vertical plane intersecting said conveyor belt, means biasing said second pressing roller into engagement with said conveyor belt to press at least that portion of metallic plates disposed to one side of the longitudinal center line of said passage against the base surface of said passage, said second pressing roller engaging said conveyor belt at a detecting position line extending perpendicularly to the direction of movement of metallic plates along said passage, a pair of electromagnetic detecting windings positioned beneath said passage on said detecting position line and disposed symmetrically on opposite sides of the longitudinal center line of said passage, said second position detector being operable to detect entry of metallic plates beneath said second pressing roller, and signal means including said electromagnetic windings and said second position detector operable responsive to detection of entry of a metallic plate beneath said second pressing roller to produce an output signal which is a function of the difference between the respective distances of the metallic plate from said two electromagnetic windings resulting from deformation of the metallic plate by said first-mentioned pressing roller.

3. A material discriminating device for metallic plates, as claimed in claim 2, in which said second position detector generates a sampling pulse responsive to a metallic plate passing over said detecting position line, said signal means including a bridge circuit having said electromagnetic detecting windings connected in a differential relationship, and a discriminator circuit generating an output signal responsive only to simultaneous existence of said sampling pulse and an output signal of said bridge circuit exceeding a predetermined signal level.

4. A material discriminating device for metallic plates, as claimed in claim 3, in which said signal means further includes an AND circuit having an input connected to said discriminator circuit and an input connected to said position detector.

5. A material discriminating device for metallic plates, as claimed in claim 1, including a second material discriminating device detecting the electrical resistance of metallic plates by electromagnetic induction and comparing the detected value with a reference resistance value.

6. A material discriminating device for metallic plates, as claimed in claim 1, in which said pressing means comprises a pair of rollers in mutually opposing relation and including a rotatable plate support roller mounted at a fixed position and a rotatable plate pressing roller mounted for vertical displacement, means rotating said pressing roller in the plate travel direction, resilient means biasing said pressing roller toward said support roller, electromagnetic means operatively connected to said pressing roller and selectively energizable to press said pressing roller against metallic plates at said first point along said passage and engaged by said support roller to impart a flexural deformation to the metallic plates, said first position detector being operable to detect entry of metallic plates between said rollers, and current control means connected to said electromagnetic means and to said first position detector and operable responsive to detection of a metallic plate entering between said rollers to energize said electromagnetic means momentarily to effect flexural deformation of the metallic plate.

7. A material discriminating device for metallic plates, as claimed in claim 6, including vertically reciprocal mounting means rotatably mounting said pressing roller, said electromagnetic means including a magnetic stator, a magnetic armature connected to said mounting means and movable relative to said stator and an energizing winding wound on said stator and connected to said current control means. 

1. A material discriminating device for metallic plates comprising an elongated base member having a passage of such dimensions as conform to configuration of plates whose material is to be discriminated as the plates are moved along said passage, pressing means positioned at a first point along said passage and operable, only when activated, to apply a predetermined load to metallic plates passing said first point, deformation detecting means positioned at a second point along said passage and operable, only when activated, to detect the magnitude of any permanent deformation of the metallic plates, passing said second point, caused by said pressing means, first plate detector means operable, responsive to detection of a plate at said first point, to activate said pressing means only when a plate is passing said first point, and second detector means operable responsive to detection of a plate at said second point, to activate said deformation detecting means only when a plate is passing said second point, whereby the material of metallic plates moving along said passage may be discriminated on the basis of the magnitude of the permanent deformation which depends upon the elastic limit of the metallic plates.
 2. A material discriminating device for metallic plates, as claimed in claim 1, in which the base surface of said passage is substantially flat and planar, said deformation detecting means comprising a conveyor belt exTending along said passage at a location spaced substantially laterally to one side of the longitudinal center line of said passage, said conveyor belt being engageable with metallic plates to move the same along said passage; a second pressing roller mounted for rotation in a vertical plane intersecting said conveyor belt, means biasing said second pressing roller into engagement with said conveyor belt to press at least that portion of metallic plates disposed to one side of the longitudinal center line of said passage against the base surface of said passage, said second pressing roller engaging said conveyor belt at a detecting position line extending perpendicularly to the direction of movement of metallic plates along said passage, a pair of electromagnetic detecting windings positioned beneath said passage on said detecting position line and disposed symmetrically on opposite sides of the longitudinal center line of said passage, said second position detector being operable to detect entry of metallic plates beneath said second pressing roller, and signal means including said electromagnetic windings and said second position detector operable responsive to detection of entry of a metallic plate beneath said second pressing roller to produce an output signal which is a function of the difference between the respective distances of the metallic plate from said two electromagnetic windings resulting from deformation of the metallic plate by said first-mentioned pressing roller.
 3. A material discriminating device for metallic plates, as claimed in claim 2, in which said second position detector generates a sampling pulse responsive to a metallic plate passing over said detecting position line, said signal means including a bridge circuit having said electromagnetic detecting windings connected in a differential relationship, and a discriminator circuit generating an output signal responsive only to simultaneous existence of said sampling pulse and an output signal of said bridge circuit exceeding a predetermined signal level.
 4. A material discriminating device for metallic plates, as claimed in claim 3, in which said signal means further includes an AND circuit having an input connected to said discriminator circuit and an input connected to said position detector.
 5. A material discriminating device for metallic plates, as claimed in claim 1, including a second material discriminating device detecting the electrical resistance of metallic plates by electromagnetic induction and comparing the detected value with a reference resistance value.
 6. A material discriminating device for metallic plates, as claimed in claim 1, in which said pressing means comprises a pair of rollers in mutually opposing relation and including a rotatable plate support roller mounted at a fixed position and a rotatable plate pressing roller mounted for vertical displacement, means rotating said pressing roller in the plate travel direction, resilient means biasing said pressing roller toward said support roller, electromagnetic means operatively connected to said pressing roller and selectively energizable to press said pressing roller against metallic plates at said first point along said passage and engaged by said support roller to impart a flexural deformation to the metallic plates, said first position detector being operable to detect entry of metallic plates between said rollers, and current control means connected to said electromagnetic means and to said first position detector and operable responsive to detection of a metallic plate entering between said rollers to energize said electromagnetic means momentarily to effect flexural deformation of the metallic plate.
 7. A material discriminating device for metallic plates, as claimed in claim 6, including vertically reciprocal mounting means rotatably mounting said pressing roller, said electromagnetic means including a magnetic stator, a magnetic armature connected to said mounting means and movable relative to said stator anD an energizing winding wound on said stator and connected to said current control means. 